The present application is based on Japanese Application Nos. Hei. 10-147690, Hei. 10-219303 and Hei. 10-367618, which are incorporated herein by reference.
The present invention relates to a support for a lithographic printing plate, a production method thereof and a polishing method of an aluminum plate. More specifically, the present invention relates to an aluminum support for a lithographic printing plate, in which the surface of the aluminum support is uniformly roughened, so that when a lithographic printing image is formed on the surface, good suitability for plate inspection is attained, high printing durability is obtained due to good adhesion between the aluminum support and the photosensitive layer, good water receptivity is assured, and the amount of fountain solution on the plate surface at the printing can be easily viewed.
In the field of lithographic printing plates, it is well known to produce a lithographic printing plate by coating a positive or negative photosensitive composition on the surface of an aluminum support and then subjecting the composition to drying, exposing by a chemical radioactive ray and developing.
In general, the aluminum plate is necessary to pass through several treatments before it is actually used as a support of a lithographic printing plate. For example, the aluminum plate is treated by a combination of one or more of mechanical surface graining, chemical etching, electrochemical surface graining and the like to impart water receptivity and adhesion to the photosensitive layer. The aluminum surface not subjected to anodization is fairly soft and easily abraded, therefore, the surface is usually anodized to form an oxide coating thereon. The thus-treated aluminum plate has a hard surface and is favored with excellent abrasion resistance, high affinity for water, good holding property and strong adhesion to the photosensitive layer. Thereafter, the surface is generally sealed with a composition for imparting water wettability and then a photosensitive composition is coated thereon.
As the surface graining method of an aluminum support for a lithographic printing plate, AC etching is commonly used, where the current used is a special alternating waveform current such as normal sinusoidal current and rectangular wave, and an AC current is passed using an appropriate electrode such as graphite as a counter electrode to thereby roughen the surface of an aluminum plate. The treatment is usually performed once but the pit depth obtained is shallow all over and the plate has poor printing durability. To cope with this, a number of methods have been proposed with an attempt to obtain an aluminum plate suitable as a support for printing plate, which is grained such that pits having a depth larger than the diameter are uniformly and densely present. For example, a surface graining method using a special electrolysis power source waveform (see, JP-A-53-67507 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)) and methods characterized by the ratio in the quantity of electricity between anode time and cathode time at the electrolysis surface graining using alternating current (see, JP-A-54-65607), the power source waveform (see, JP-A-56-25381) or the combination of the quantities of electricity passed per unit area (see, JP-A-56-29699) are known.
On the other hand, an aluminum support is produced by such a method that an aluminum ingot is dissolved and held to produce a slab (thickness: from 400 to 600 nm, width: from 1,000 to 2,000 mm, length: from 2,000 to 6,000 mm) and the slab is passed through a scalping step of cutting the impurity structure part on the slab surface by a scalping machine at every 3 to 10 mm and then a soaking treatment step of holding the slab in a soaking pit at from 480 to 540xc2x0 C. for from 6 to 12 hours for removing the stress inside the slab and uniformalizing the structure. Thereafter, the aluminum plate is rolled by hot rolling at from 480 to 540xc2x0 C. into a thickness of from 5 to 40 mm, cold rolled at room temperature into a predetermined thickness, annealed for uniformalizing the structure to homogenize the rolled structure and the like, cold rolled into a prescribed thickness, and then remedied to obtain a sheet having good flatness. The thus-prepared aluminum support is used as a support for a lithographic printing plate.
In view of energy savings or effective use of resources, it is demanded to use a general-purpose aluminum plate or an aluminum plate produced by omitting the intermediate annealing or soaking from the production process of the aluminum plate, as the aluminum support for a lithographic printing plate.
However, if an aluminum support for a lithographic printing plate is manufactured from such an aluminum plate, treatment unevenness called streaks or grainy unevenness is readily generated. This is considered to occur because the chemical dissolution reaction of aluminum proceeds at a different rate depending on the crystal orientation and in turn the electrochemical pitting reaction of aluminum proceeds differently depending on the crystal orientation.
In other words, irregularities generated due to the difference in the dissolution rate at the chemical dissolution reaction appear as streaks or grainy unevenness or the difference in the pitting reaction depending on the crystal orientation (difference in the pit number or size) appear as streaks or grainy unevenness.
In order to improve the retardation to scumming at the printing when a printing plate is manufactured, it is well known to perform chemical etching after the surface graining. When the plate is used as a PS plate for proof printing, disinclination for catching of a sponge supplying fountain solution is one of important printing capabilities. When the sponge is difficult to catch, debris of the sponge is scarcely generated and the printing plate is not prone to scumming. The increase in the etching amount after the surface graining is accompanied with the following advantage and disadvantage.
When the amount of chemical etching is increased, the printed matter is difficultly soiled, however, due to the difference in the aluminum dissolving rate ascribable to the crystal orientation of aluminum, streaks or grainy unevenness become outstanding. Furthermore, adhesion between the photosensitive layer and the aluminum support is deteriorated to reduce the printing durability.
In general, an aluminum support for a lithographic printing plate is subjected to anodization after the surface graining to have white appearance which is considered preferable, so that when a lithographic printing image is formed thereon, good visual contrast can be attained between the image area and the non-image area and the evaluation of the image quality by a printer can be facilitated. To this effect, the aluminum plate is demanded to have a white surface so as to obtain good image contrast and at the same time to ensure an image adhesion on the surface of the sheet subjected to surface graining and anodization and also have a surface hardness.
On the other hand, in general, an aluminum plate used as a lithographic printing plate support is required to have appropriate adhesion to the photosensitive material and appropriate water receptivity and also must be uniformly surface grained. The term xe2x80x9cuniformly surface grainedxe2x80x9d as used herein means that pits produced are appropriately uniform in the size and such pits are produced uniformly throughout the surface. The pit has an outstanding effect also on the printing performance of the printing material, such as disinclination for scumming and printing durability, and the pit quality is an important factor in the production of a printing material. In recent years, it is a problem to be solved to continuously and stably produce good quality pits.
JP-A-6-92052 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) has proposed a patent constituted by a step of mechanical surface graining, a step of etching of from 0.5 to 30 g/m2 and a step of pulse passing of from 200 to 600 c/dm2. JP-A-7-9776 has proposed to perform mechanical surface graining, etching of from 1 to 5 g/m2 and then electrochemical surface graining with a quantity of alternating current (AC) electricity of from 300 to 800 c/dm2. Also, a patent constituted by mechanical surface graining, etching of from 0.5 to 30 g/m2 and ac electrolysis of from 200 to 600 c/dm2 has been proposed. The surface graining of the support surface includes mechanical surface graining, chemical surface graining and electrochemical surface graining. In this concern, JP-A-6-24166 discloses a patent where various conditions for the mechanical surface graining, chemical etching and electrochemical surface graining are varied, more specifically, after the mechanical surface graining, chemical etching of from 0.5 to 30 g/m2 is performed, electrochemical surface graining is applied at an appropriate current density with an appropriate quantity of electricity, etching of from 0.1 to 10 g/m2 is performed to finish smooth angles, and then anodization is applied.
However, as a requirement from clients in recent years, a high-quality printing plate more freed from local unevenness is demanded. Therefore, a lithographic printing plate support capable of satisfying such a requirement is demanded and at the same time, the production cost must be minimized. According to the techniques disclosed in JP-A-692052 and JP-A-6-24166, preliminary polishing is omitted, accordingly, when an original AL having local recessions is subjected to mechanical surface graining, chemical etching and electrochemical surface graining, the recession part fails in attaining uniform chemical etching or electrochemical surface graining and troubles are generated in that portion, such as reduction in the printing durability or deterioration in the disinclination for catching up of ink on the recession.
Under these circumstances, the present invention has been made. Accordingly, the object of the present invention is to provide a method for producing a high-quality lithographic printing plate support free of local unevenness.
As a result of extensive investigations, the present inventors have found that when an aluminum plate is subjected to polishing after the surface graining, the Projected portion is roundly shaved and the disinclination for catching of a sponge and in turn the scumming reduction can be improved.
Furthermore, the present inventors have found a production method of an aluminum support for a lithographic printing plate, such that by subjecting an aluminum plate treated to have fine irregularities to electrochemical surface graining in an acidic aqueous solution and then to polishing, an aluminum support prevented from generation of troubles called streaks or grainy unevenness ascribable to the aluminum crystal orientation and not prone to scumming can be obtained.
When fine irregularities are imparted to an aluminum plate, the fine irregularities cause irregular reflection and thereby the streak or grainy unevenness becomes difficult to recognize. Moreover, the presence of fine irregularities have an effect that honeycomb pits of from 0.1 to 3 xcexcm are thereafter uniformly produced in an aqueous nitric acid solution independent of the crystal orientation, as a result, the streak or grainy unevenness becomes difficult to appear.
Further, the above-described object of the present invention can be attained by a production method of a lithographic printing plate support, which comprises subjecting a lithographic printing plate support to surface graining or etching by mechanical surface graining, chemical etching or electrochemical surface graining, and then shaving tips of irregularities, particularly projections on the surface of the lithographic printing plate support by filiform fibers having a diameter of from 5 to 600 xcexcM.
In order to solve the problems in conventional techniques, to obtain uniform quality and also to minimize the production cost, an invention descried below has been created. That is, in a method for producing a lithographic printing plate support, surface graining or etching is performed by mechanical surface graining, chemical etching or electrochemical surface graining, and tips of irregularities, particularly projections are shaved by macro filiform fibers having a diameter of from 5 to 500 xcexcm. Preferably, the fibers are formed into a roller having a hardness of 60xc2x0 or less and tips of irregularities, particularly projections on the roughened or etched surface of the lithographic printing plate support are shaved by the roller. As a result, a high-quality lithographic printing plate support free of local unevenness can be produced in the present invention.